In systems mounted on vehicles such as an anti-lock brake system and a stability control apparatus, in order to perform such controls, a rotation-detecting apparatus is utilized for measuring a rotational speed and a rotational frequency of a wheel (a rotor). Various kinds of rotation-detecting apparatuses exist. For example, some of these rotation-detecting apparatuses can not only measure a rotational speed, or a rotational frequency, but also emit output signals to which items of information, such as a rotational direction and the result of self-inspections, can be added.
For example, US2001/0002791A1 describes a conventional technique of such a kind of apparatus. This document relates to a detection signal-processing apparatus that detects a rotational direction and a rotational speed of a rotor, and emits a rotational direction signals at timings based on the rotational speed detected. The rotational direction signals have different waveforms for indicating the rotational directions detected. Thus, the rotational direction signal, or similarly, other items of additional data, can be added to the rotational speed signal.
According to the techniques described in the document, a rotational speed of the rotor is evaluated, irrespective of whether a predetermined speed is achieved or exceeded. Because rotational direction signals with a waveform corresponding to a rotational direction of the rotor are added to the rotational signal, a transfer signal, which is also different from a rotational direction and a rotational speed of the rotor, is generated in the form of a signal pattern. Therefore, only when two conditions are fulfilled simultaneously, namely, a first condition in which a rotational speed of the rotor has achieved, or exceeded, a predetermined rotational speed, and a second condition in which a signal pattern of the kind described above is exist, the transfer signal can be emitted at a timing based on the rotational speed.
According to the technique described in the document, during a period between from a time that a rotor starts to rotate, either in a normal rotational direction or in a reverse rotational direction, and a time when the rotor achieves, or exceeds, a predetermined rotational speed, because a rotational direction is normally not suddenly changed, a state of the rotational direction is assumed to remain unchanged even when outputs of the rotational direction signal are not continuously supplied. Thereafter, when the rotational speed achieves, or exceeds, a predetermined rotational speed, the rotational speed can be recognized by use of one of the rotational direction signals emitted at a timing based on the rotational speed. Accordingly, a signal pattern, generated by combining the signal waveform corresponding to the rotational direction with the rotational speed signal, is emitted as a transfer signal.
However, by use of this method, if the vehicle is not driven, in other words, if the rotor is not rotating at a level of speed which achieves, or exceeds, a predetermined speed, additional items of data can not be added to the rotational speed signal. In other words, an item of data such as an air pressure of a tire, is preferably measured when the rotor is at a stop, in other words, when the rotor is not rotating. On such occasions, however, items of data can not be added to the rotational speed signal.
On many occasions, drivers fail not only to check an air pressure of tires, but also the state of their vehicles. Accordingly, for purposes of driving a vehicle safely, it is very important to provide a rotation-detecting apparatus that incorporates the addition of items of data.
A need thus exists for a rotation-detecting apparatus in which, independently of a rotational speed of a rotor, a rotational direction signal, or an additional item of data, can be added to a rotational speed signal.